It is known to form a new well path, sidetrack, from an existing well by drilling a new well path out through the wall of a casing. This is carried out by anchoring a whipstock at a desired location in the well, whereby a drilling body, which subsequently engages the whipstock, changes direction and drills through a sidewall of the casing from the inside of the casing. Oftentimes, such a drilling operation results in an oblong opening, generally referred to as a window, in the wall of the casing. It may also be of interest, in some cases, to drill a new well path out through a window through several casing sizes disposed in a pipe-in-pipe assembly in a well. Most commonly, such a sidetrack is formed at a relatively shallow level in a well, and far away from the reservoir, so as to reasonably ensure that the casing, which is to be drilled through, is cemented to a surrounding formation, whereby the window is anchored well and becomes sufficiently stable. By so doing, no great danger exists of allowing the window portion to move during formation thereof, or during subsequent operations, for example when the drill string is being pulled back through the window. Given that the sidetrack usually is formed at a relatively shallow level in the well, the new well path oftentimes becomes very long, which incurs large costs to an operator. It would therefore have been advantageous, in many cases, if such a sidetrack could have been carried out at a deeper level in the well. At such deeper-lying locations, the/those particular casing(s), which is/are to be drilled through for the sidetrack, oftentimes is/are insufficiently cemented to the surrounding formation, and/or to each other when using several casing sizes. The cement may also have insufficient quality and/or be completely or partially absent. It may therefore be difficult, in such situations, to achieve a sufficiently good anchoring and stability of casings in order to carry out an unproblematic sidetrack operation.
Independent of how deep the particular sidetrack location is located in a well, it would have been useful if the/those particular casing(s), which is/are located at this location, and which are to be drilled through to form a window for the sidetrack, could be anchored and stabilized to a sufficient degree prior to initiating the sidetrack operation. In order to carry out such a sidetrack, it is not desirable, in this context, to remove a complete longitudinal section of the/those particular casing(s), so-called section milling, at the location of the sidetrack in the well. Besides weakening the integrity of the well by removing a complete longitudinal section of the/those particular casing(s), such a section milling is very time-consuming and costly. Moreover, such operations are encumbered with various challenges in the area of health, safety and environment (HSE).